1. Field of the Invention
The present invention relates to a coating method.
2. Description of Related Art
A coating method for coating an outer surface of a substrate such as a vehicle body generally includes a preparation step for preparing for the substrate to be coated with a paint by removing dust or other foreign materials from the substrate, a coating step for spraying the substrate with the paint, and a drying step for drying the coat thereon. The drying step generally comprises sequential setting and baking steps in particular when a thermosetting paint is used. The setting step is usually carried out prior to the baking step at an ambient temperature which is lower than the ambient temperature during the baking step, for example, at room temperature or at temperatures ranging from 40.degree. C. to 60.degree. C., in order to volatilize a solvent slowly so as to prevent a formation of pinholes on the coat surface during the baking step which is usually carried out at approximately 140.degree. C.
The substrate is held at a given position on a conveyance means such as a carriage while being conveyed during the preparation, coating, and drying steps.
A degree of flatness or smoothness on the surface of a coat on the substrate is one of standards for evaluating a quality of the coat. The higher a degree of flatness the smaller a degree of irregularities on the coat surface, thus producing a better coat. It is well known that a thicker film thickness of a paint may give a higher degree of flatness on a coat surface. A paint sprayed on the surface of a substrate may be said to sag when it is visually observed that the paint sprayed thereon flows and finds a trace of movement on the coat by 1 mm to 2 mm from a site where the paint was sprayed until it is cured in the drying step. It may be defined herein that a sag of the paint occurs if such a trace exceeds at least 2 mm when visually observed. In other words, a sagging limit thickness of a paint is a film thickness beyond the maximum film thickness at which the paint does not sag at least in the drying step if it is left as it was sprayed. Thus, a film thickness of the paint within its sagging limit thickness is a film thickness in which it does not sag in the drying step even if it is left as it was sprayed. On the contrary, a film thickness thicker than its sagging limit thickness of the paint is a film thickness at which the paint causes sagging at least during the drying step when it is stayed as it was sprayed.
The paint causes sagging when the paint coated thereon flow downwardly due to gravity. The paint becomes more likely to cause sagging as a film thickness of the paint sprayed gets thicker. Thus it is a matter of course that the paint sags more likely on a surface of the substrate extending in an up-and-downward direction, i.e., a vertically extending surface, than on a surface thereof extending in a horizontal direction, i.e., a horizontally extending surface. This enables the paint to be coated on the horizontally extending surface in a film thickness thicker than on the vertically extending surface because the sags or drips of the paint little affect adversely the coat sprayed on the horizontally extending surface of the substrate. If the film thickness of a coat on the horizontally extending surface is the same as that on the vertically extending surface, the former can produce a coat with a degree of flatness higher than the latter because the paint sprayed on the horizontally extending surface becomes flattened due to a natural flow in the paint to an extent to which no sags substantially occur.
Conventionally, in order to provide a coat with a higher degree of flatness while preventing sags or drips of a paint coated on a surface of the substrate, there have been used paints which are lower in viscosity and less flowable. Even if such a thermosetting paint is used, however, a sagging limit thickness of the paint sprayed on the vertically extending surface is as high as approximately 40 .mu.m. This sagging limit thickness is the maximum film thickness in which the paint does not substantially sag on the vertically extending surface of a substrate. In other words, the paint is likely to sag or drip in initial stages of the setting and baking steps, particularly in the initial stage of the baking step. Accordingly, a film thickness of the coat is determined by a film thickness of the paint to be sprayed on the surface of a substrate to such an extent that the paint does not sag on its vertically extending surface. In order to produce a coat in a film thickness thicker than a sagging limit thickness of the paint, the spraying step is repeated twice or more in conventional coating method.
Attempts have been made to complete with the problem with spraying the paint in a film thickness thicker than its sagging limit thickness, and we have developed a technology that enables forming a coat having coat properties superior to coats obtainable by conventional coating methods, when sprayed in the same film thickness, as disclosed in our U.S. Pat. application Ser. No. 100,767, now U.S. Pat. No. 4,874,639 This technology involves spraying a vehicle body with the paint in a film thickness thicker than its sagging limit thickness and rotating the body about its substantially horizontal axis at least until the paint in the coat sprayed thereon is cured so as to cause no sagging any more. This coating method rather takes advantage of gravity that causes sags of the paint sprayed and the substrate is rotated so as to alter its direction in which gravity acts on the coat surface on the body, thereby preventing sags from occurring in the coat thereon while positively utilizing a flowability inherent in the paint and yielding a coat with a higher degree of flatness than coats obtainable by conventional coating methods. Thus this technology is an excellent coating method in itself.
It has now been found, however, that there is still a room for improvement in the above-described technology because, when a highly flowable paint is sprayed in a film thickness thicker than its sagging limit thickness, the paint sprayed on the substrate may swell on an edge portion thereof to form a mass of the paint. As a result of review on this problem, it has been found that the highly flowable paint sprayed gets flattened by means of a surface tension acting upon the coat surface, but the paint, once flattened, then moves in one direction toward the edge portion of the substrate by means of a surface tension, forming a swelled mass of the paint.